Due to their outstanding heat and weather resistance and light weight, silicone rubber sponges are used for automotive parts, such as packings, gaskets, and O-rings; as sheath materials for rollers in copiers, and as sealing materials of various kinds. A number of silicone rubber sponge compositions have been proposed to date. Patent Publication 44-461 and Unexamined Patent Application 7-247436, for example, teach silicone rubber sponge compositions containing thermally decomposing organic blowing agents such as azobisisobutyronitrile. However, these compositions give rise to harmful decomposition products during sponge production and thus pose an environmental problem. Patent Publication 7-122000 proposes a silicone rubber sponge composition consisting of an oil-in water type emulsion of dimethylpolysiloxane, an emulsifier, water, and a thickener. However, this composition does not readily give silicone rubber sponges with uniform cells. Further, since oil-in water type emulsions and silicone rubber compositions are basically immiscible, the process of evenly dispersing the silicone rubber composition throughout the composition is time consuming, and throughput is poor.
It is an object of the present invention to provide a silicone rubber sponge composition that does not give rise to harmful decomposition products during sponge production and that with curing gives a silicone rubber sponge having fine, uniform cells.